The present invention relates to an electrically energised device for coalescing a first conductive fluid, emulsified in a second fluid, comprising: a tube member, having at least one fluid inlet and at least one fluid outlet, said tube member defining a flow channel for an emulsion of the first and second fluid from its inlet side to its outlet side and comprising at least one electrically insulating layer portion, said channel having a generally circular or elliptic cross-section, and an interacting pair of a first and a second electrode that are arranged outside and adjacent to the at least one insulating layer portion, that separates the electrodes from the channel and thereby from immediate contact with the emulsion, and supplied with a pulsed or alternating voltage for the purpose of subjecting the first and second fluid flowing through the flow channel to an electrostatic field.
Since the invention is particularly applicable to electrostatic coalescer devices for promoting the coalescense of water in an emulsion comprising oil and water, it will be described with reference to such an application. However, it should be understood that it is applicable to all kinds of coalescer applications in which there is possible to affect and promote the coalescense of a first fluid in an emulsion comprising a first and a second fluid by means of an electric field applied to the emulsion.
Although the invention is applicable to all kinds of oil-treatment lines, it is particularly advantageous in off-shore applications in which a coalescer is arranged for the purpose of promoting or effectuating a pre-separation of water from oil, or a water droplet enlargement, before an extracted emulsion comprising oil and water is further conducted to a subsequent settling tank for gravitational settling of the oil. In such an application the coalescer contributes to an important reduction of the overall weight of the settling arrangement.
In the oil industry where oil is extracted from one ore more wells in an oil field oil will be extracted together with water. The water has to be removed from the oil and this is mainly done by means of settling tanks in which the oil is permitted to settle through the action of the gravitational force. However, complex oil-water emulsions may develop during the extraction of the oil. For example, the removal of gas from the oil-water emulsion by means of gas-liquid cyclones might contribute to a more complex emulsion, which will be difficult to separate only by means of settling.
It is well known to arrange so called electrostatic coalescers for the purpose of obtaining a destabilisation of water-in-oil emulsions, that is a water droplet enlargement or coalescense of water in the oil.
U.S. Pat. No. 4,601,834 discloses an electrostatic coalescer in which one of the electrodes is arranged at the inner periphery of a duct-defining tube, while the other electrode is located at the outer periphery of the same tube on the opposite side thereof. Such an arrangement has the disadvantage that the inner electrode has to be covered with an insulating material, and that the inner electrode will also be a physical hinder for the emulsion flowing in the duct through the coalescer. However, contemporary coalescers with a circular or elliptic channel cross-section still present this somewhat disadvantageous design.
In a publication by O. Urdahli et al, entitled xe2x80x9cElectrostatic destabilisation of water-in-oil emulsions under conditions of turbulent flowxe2x80x9d, Trans IchemE, vol 74, Part A, March 1996, p. 158-165, there is disclosed an electrostatic coalescer the electrodes of which are located on opposite sides of and outside a tube that defines a flow channel with a rectangular cross section. The electrodes are parallel plates which are insulated from the emulsion by an insulating wall of the tube of the coalescer. A number of electrode pairs are arranged in the length direction of the tube. However, a disadvantage by such an arrangement is the non-optimised distribution of the electric field over the cross-section of the emulsion flow channel. Moreover, for full-scale applications, a rectangular channel is not practical. However, a channel with a circular or elliptic cross-section would be preferred instead.
It is an object of the invention to provide an electrically energised coalescer device with active electrodes that are saved from direct contact with an emulsion that is treated by means of the device. The construction of the device shall promote an optimised distribution of an electric field over the cross-section of the emulsion flow channel. The device shall also be operatively reliable and shall promote an easy maintenance thereof.
The object of the invention is obtained by means of the initially defined device, characterised in that the first and second electrodes are separated with a gap and extend continuously generally in parallel in the length direction of or helically around the flow channel. Thereby an advantageously distributed electric field is obtained in the flow channel through which the treated emulsion is flowing.
Extending in parallel in or with the length direction of the channel corresponds to the extreme case of a helix with an infinite pitch. In such a case it is preferred that the respective electrodes are sheet-like and present an inner curvature that is generally parallel to the adjacent inner periphery of the tube member, i.e. to the adjacent outer periphery of the channel. Each such electrode may then cover or enclose a sector of the channel. The electrodes then define two layers or linings, arranged at opposite sides of the channel and enclosing at least a part of the cross section of the channel.
Preferably, the flow channel defined by the inner periphery of the tube member has a generally circular cross-section. In full-scale applications for the treatment of oil-water emulsions, this is an advantageous design, and the double helical arrangement of the electrodes will result in a particularly well distributed electric field in a channel with such a cross section.
The tube member is preferably generally cylindric, the electrodes being either attached to the outer periphery of the tube member or being located inside the wall of the tube member. In the latter case the electrodes are preferably moulded into the tube member wall or inserted into pre-arranged recesses therein.
The tube member preferably comprises an inner layer adapted to be in direct contact with the emulsion of first and second fluid flowing in the flow channel and being able to withstand high electrical fields, an intermediate layer in direct contact with the electrodes and being able to withstand high electrical fields, and an outer layer. The material of one of the layers should be adapted to withstand the pressure load better than the materials of the other layers. The inner layer material should be chemically compatible with the emulsion and resistant against high electric fields. Examples of such a material are PEEK, Plexiglas or a thermoplastic. The main constituent of the intermediate layer is preferably a curable resin, such as epoxy, or a polymer composite material while the main constituent of the outer layer preferably is a polymer or a fibre-reinforced polymer or steel. The electrodes are preferably located in the intermediate layer.
Further features and advantages of the present invention are presented in the following detailed description and the further dependent claims.